A valve timing control apparatus is known which appropriately adjusts an opening and closing timing of a valve, and achieves optimal driving conditions by means of a displacement of a relative rotational phase between a driving side rotational member, which rotates in synchronization with a crankshaft, and a driven side rotational member, which rotates in synchronization with a camshaft. A known valve timing control apparatus is disclosed in JP2002-097912A (see pp.2–3 FIGS. 2–5).
The disclosed valve timing control apparatus includes a housing, a rotor, and a vane. The housing rotates in synchronization with the crankshaft, the rotor is relatively rotatably engaged with the housing to form a fluid pressure chamber between the housing and the rotor, and synchronously rotatable with the camshaft, and the vane is provided at the housing or the rotor to separate the fluid pressure chamber into a retarded angle chamber and an advanced angle chamber. The valve timing control apparatus further includes a first passage for operating a relative rotational phase between the housing and the rotor within a range from a most retarded angle phase to a most advanced angle phase, a locking member for locking the relative rotational phase at an intermediate phase within a range from the most retarded angle phase to the most advanced angle phase, a spring for operating the locking member in a locking direction, and a relative rotation controlling mechanism including a locking fluid passage for operating the locking member in an unlocking direction against a biasing force of the spring.
According to this valve timing control apparatus, when rotational speed of the engine is increased, because of a centrifugal force applied to the locking member, the locking member is unlocked against a biasing force of the spring. In view of the above mentioned considerations, in order to make the locking member less likely to be unlocked caused by the centrifugal force due to a rotation of the engine, when the relative rotational phase between the housing and the rotor is locked at a predetermined intermediate phase, the valve timing control apparatus supplies fluid into one of the retarded angle chamber and the advanced angle chamber, and drains the fluid from the other of the retarded angle chamber and the advanced angle chamber, and hence the locking member generates a frictional resistance force (i.e., a resistance in the unlocking direction). Accordingly, with the configuration of the valve timing control apparatus disclosed in JP2002-097912A, because a fluid pressure of the one of the retarded angle chamber and the advanced angle chamber is applied to the vane, a biasing force in one direction is applied to the vane. In consequence, in a condition where the relative rotational phase is locked, the locking member and a mating wall surface are relatively pressed and a friction therebetween is increased. Accordingly, the resistance force is increased and the locking member is thereby less likely to shift in a centrifugal direction.
According to the valve timing control apparatus disclosed in JP2002-097912A, in order to prevent the locking member from being unlocked, the fluid pressure is necessarily supplied into the one of the retarded angle chamber and the advanced angle chamber. However, because the fluid pressure is supplied by means of a pump, which is activated by a driving force of the engine, immediately after an engine starting, the fluid pressure from the pump cannot be reached the one of the retarded angle chamber and the advanced angle chamber, thus the valve timing control apparatus less likely to supply a sufficient fluid pressure to the one of the retarded angle chamber and the advanced angle chamber. Therefore, if a control is performed for rapidly increasing the rotational speed of the engine immediately after the engine starting, because the fluid pressure is not yet sufficiently supplied into the one of the retarded angle chamber and the advanced angle chamber, the centrifugal force is applied to the locking member before the friction force between the locking member and the mating wall surface is increased, and the locking member may thereby occasionally be unlocked without difficulty.
A need thus exists for a valve timing control apparatus, which, even in a condition where the sufficient fluid pressure is not yet supplied to the valve timing control apparatus immediately after the engine starting, prevents the locking mechanism from being in an unlock state because of the centrifugal force caused by an increase of the rotational speed of the engine.